A common lock used on doors is a safety chain which allows a door to be partially opened so that a visitor may be viewed before being admitted, or so that articles, such as mail, may be passed through the partially-opened door. A safety chain includes a mounting block which is attached securely to the door jamb and receives one end of a chain, or a variant of this such as a strap or cable. Another element is securely mounted to the door and forms an elongate slot for receiving a second end of the chain. This element is oriented with the slot horizontal and spaced from the mounting block such that the distance from the mounting block to the remote end of the slot is substantially equal to the length of the chain. When the door is opened, the second end of the chain slides to the end of the slot nearest the mounting block, thus providing slack in the chain to permit the door to be slightly opened.
While the safety chain of the prior art has been generally accepted as providing good security, it has been shown not to be fail-safe. In fact, burglars have become quite adept at circumventing the prior art safety chain. For example, in a hotel it is common for keys to rooms to be freely available to those who want them. Keys are frequently dropped on the street and picked up by persons who then pass them to prospective burglars. Furthermore, the staff of a hotel is large enough that one who desires to have keys to rooms can easily obtain them. Once the burglar has a key to the room, the only impediment to entry is frequently the safety chain as described above.
One technique which has been developed for avoiding the safety chain is to open the door and engage the second end of the chain with a coat hanger. The door is then closed and the coat hanger manipulated to slide the second end along the slot so that the second end falls out of the end of the slot, thus freeing the safety chain. Another technique is to utilize a coat hanger or other implement while the door is open to engage a rubber band between the second end of the chain and a remote end of the slot element. Then, it is only necessary to close the door and allow the rubber band to pull the second end along the slot and allow it to fall free.